In co-injection methods, a co-injection manifold receives material, usually thermo-plastic, from two different injection units and combines the two materials into a single stream that flows into a mold or die. The manifold allows one material to be co-molded inside another during a single cycle. The co-injection manifold is thus located between the injection units and the mold. A typical co-injection manifold is fixed to the injection units.
Exothermic-blowing agents have been incorporated into thermo-plastics in mono-injection-molding methods and in other injection molding methods. A blowing agent is a substance incorporated into a mixture for the purpose of producing a foam. But the use of exothermic-blowing agents in mono-injection-molding methods has been limited to the production of end products, for which aesthetic concerns do not apply. In other words, using exothermic-blowing agents in conjunction with co-injection methods yields aesthetic and other problems for the following reasons.
First, using exothermic-blowing agents in the core material produces uneven blowing, which may cause voids in certain inner areas of the parts being produced. Because the blowing agent is exothermic, heat evolves unevenly during the foaming reaction, which accounts for the uneven blowing and the formation of voids in the core material.
Second, and somewhat related, it is difficult to keep the exothermic-blowing agent homogenized within the core material during production. As a result, when the exothermic-blowing agent is mixed with the inner-core material, a heterogeneous mixture tends to form, making it difficult to produce a high quality end product. For example, one part cycle may produce a part exhibiting high pockets of potency while a subsequent part cycle may produce a part exhibiting weaker pockets of potency. Keeping the blowing agent homogenized is critical to providing consistent “part feel” and structural integrity to the end product. “Part feel” describes the physical characteristics of the product, including how a part feels when touched by the hands of a consumer. Overall, it is difficult to regulate the amount of and homogenize the liquid-exothermic-blowing agent being introduced into the core material between part cycles while producing products.
Third, using exothermic-blowing agents results in irregular and inconsistent pressure being exerted on the outside walls of the part after removal from the die. Consequently, part size and shape are affected and many parts generated therefrom need to be scrapped. Controlling the pressure that the blowing agent exerts on the outside walls of the part after it is removed from the mold produces a part having a rounder, friendlier “part feel.” A related problem associated with adding exothermic-blowing agents to core materials is that these blowing agents tend to migrate toward the surface or skin of the part, thereby affecting the aesthetic appearance of the surface. This has limited the use of exothermic-blowing agents for application in parts that are not aesthetically demanding.
Finally, when exothermic-blowing agents are used, slight temperature changes and variations result in complications in part sizes and part cycles. In other words, the exothermic-blowing agents are not very versatile, and require uniform, constant temperatures to produce a consistent, quality end product. Minor temperature changes and modifications are often required during coring processes to overcome aesthetic, size, or cycle issues. As a result, the use of exothermic-blowing agents in these injection molding methods is limited.